In the same work, he is revealed to be the same character as "the Necromancer" from Tolkien's earlier novel The Hobbit. In Tolkien's The Silmarillion (published posthumously by Tolkien's son Christopher Tolkien), he is also revealed to have been the chief lieutenant of the first Dark Lord, Morgoth. Tolkien noted that the "angel" powers of his constructed myth "were capable of many degrees of error and failing", but by far the worst was "the absolute Satan rebellion and evil of Morgoth and his satellite Sauron."

The cosmological myth prefixed to The Silmarillion explains how Eru, "the One", initiated his creation by bringing into being innumerable spirits, "the offspring of his thought," who were thus with him before anything else had been made. The being later known as Sauron thus originated as an "immortal (angel) spirit." In his origin, Sauron therefore perceived the Creator directly. As Tolkien noted: "Sauron could not, of course, be a 'sincere' atheist. Though one of the minor spirits created before the world, he knew Eru, according to his measure."

In the terminology of Tolkien's invented language of Quenya, these angelic spirits were called Ainur (sg. Ainu). Those who entered the physical world were called Valar (sg. Vala), especially the most powerful ones. The (relatively) lesser beings of the same race, of whom Sauron was one, were called Maiar (sg. Maia). In Tolkien's letters, the author noted that Sauron "was of course a 'divine' person (in the terms of this mythology; a lesser member of the race of Valar)". Though less mighty than the chief Valar, he was more powerful than many of his fellow Maiar; Tolkien noted that he was of a "far higher order" than the Maiar who later came to Middle-earth as the WizardsGandalf and Saruman.